Apparently you are unaware of the mountains of scientific research available that detail our ability to control our subconscious mind.
It is a willing, obedient and faithful servant, unable to make a distinction between a negative or positive thought.
A simple exercise is to tell yourself, or someone else, NOT to think of a horse, or some other such idea.
Our ability to direct our thoughts (especially powerful when you add emotion) shapes our present and future. We CAN change our thought patterns, and the resulting behavioural changes affects our actions and results.
Most people are so tuned out of conscious awareness of their actions (habits) that they establish beliefs that tend to reinforce themselves.
A child, told that they are worthless or stupid will accept that information as they haven’t acquired the intellectual maturity to filter out harmful information.
There are numerous reasons to accept positivity over negativity.